Comment 13 for bug 1662465

The attached patch fixes
- the original issue spotted in this bug (ampersand "&" in the filename which was resulting in an invalid title of the internally generated SVG document holding the symbols). The title is now properly escaped.
- the issue with non-ANSI filenames on Windows (i.e. German umlauts / Cyrillic characters). I applied the same fix as in bug #1416949.
- a third issue caused by invalid symbol IDs. Before it was possible to create IDs that were invalid XML (preventing to load the symbol file to start with) but it was also possible to create IDs that were accepted by the parser while resulting in invalid hrefs when actually using a symbol (therefore loading the symbols succeeded but as when attempting to add them to the document they were not visible due to the invalid href). With the patch all characters that do not match [a-zA-Z0-9_-] are replaced with an underscore "_") which should result in valid IDs in almost all cases.

@su_v: could you check whether this fixes the issue on OS X? If not we have to investigate further...